1. Technical Field
The present invention relates to a high-field motor, such as a vehicle motor, which requires high torque, and more particularly to a double-stator motor configured by arranging an annular rotor in which radially magnetized permanent magnets are each embedded in the center portion of a pole, a three-phase inner stator arranged in a radially inward portion of the rotor and a three-phase outer stator arranged in a radially outward portion of the rotor.
2. Related Art
Mainstream motors in recent years are synchronous motors embedding neodymium magnets as permanent magnets. For such neodymium magnets, a heavy rare earth metal, such as dysprosium, is indispensable as an additive material. However, with the soaring prices of heavy rare earth metals, future use of such rare-earth magnets in mass-produced motors will carry a risk. For this reason, research and development are being accelerated for the production of motors which can reduce use of rare-earth magnets as much as possible.
In particular, in the art of motors for driving vehicles (axle motor, in particular, used in plural numbers), a large torque is desired to be generated using a small-size motor. Therefore, it is very important in the art to provide a motor structure that can exert high resistance against demagnetization as much as possible. Conventional art related to an axle motor can be seen, for example, in the double-stator motor disclosed in JP-A-2007-282331.
A well-known double-stator motor based on conventional art takes a measure against demagnetization by increasing the thickness of each rare-earth magnet that has high resistance against demagnetization. Thus, using a large amount of magnet, sufficient yield strength is ensured in the magnetic field of the stator. However, such a double-stator motor unavoidably increases the number of poles. Moreover, when such a double-stator motor is used as an axle motor, the amount of magnet used for one vehicle becomes extraordinarily large because an axle motor is needed for each of the axles.